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A note on post-harvest physiology and storage of Nigerian crops
R. Orraca-Tetteh*
The need to produce enough food for the rapidly growing populations of developing countries is more urgent than ever before, and with population increments of 3 per cent in some of them, food production has lagged far behind food needs
Attempts have been made to increase food production by putting more land under cultivation and by the use of improved seeds, more fertilizer, and pesticides. However, if examined closely, some of the apparent gains in food production have not been actual gains Great losses are sustained from the moment the food crop is ready to be harvested until it reaches the consumer's table. These can be attributed to two major causes: infestation by microorganisms, insects, and rodents; and the fact that crops are living plant tissues which during storage undergo metabolic and physiological activities that utilize nutrients, these activities being adversely affected by various environmental conditions with resultant large losses of food
1. Root and Tuber Crops
The most important root and tuber crops in tropical Africa are yams, cassava, sweet potatoes, and cocoyams Yams are usually stored in open areas after harvesting, with very little protection from the elements. The yam tuber consists of living tissue, and although these tissues are dormant at the time of harvesting, some metabolic activities continue at a slower pace during storage. Respiration in the tissues utilizes the stored food material, which is mainly carbohydrate, with the evolution of carbon dioxide and water vapor.
Higher temperatures quicken the respiratory processes by bringing dormant tissues back to activity. In general, for every 10-degree rise in temperature, there is a doubling of the respiratory rate in plant tissues Thus, with the high temperatures prevalent in tropical countries, respiration is the major source of weight loss in stored yams and food loss can be extremely high.
The stacking of yams in open barns, as practiced in West Africa, provides good ventiliation for the tubers so that temperatures are almost uniform. Experiments carried out on the respiration of slices of yam tuber showed that at temperatures between 20 and 30° C there was a rapid uptake of oxygen, but this rate was slowed markedly at temperatures between 5 and 10 C.
Microorganisms that gain entry into the tuber are another cause of storage loss in yams This usually occurs if the tuber is damaged or even bruised during harvesting or storage. Various microorganisms, such as Botryodiplodia theobromae (Pat) and Fusaria sp, invade the tissues and can, within a few days, cause the whole tuber to become a brown, rotten mass. The infection of yam tubers with microorganisms has also been found to increase the rate of respiratory loss.
The importance of such microorganisms has received comparatively little attention for a crop of such economic value, particularly in West Africa.
Another important cause of weight loss in yams is the loss of water through evaporation from the tuber. The high temperatures and low humidity during the harmattan months, when a dry, hot wind blows off the Sahara Desert, dry out the yams Weight changes in stored yams due to loss of water can amount to 5 to 10 per cent. Earlier processing of yams can reduce these preventable losses and provide more food.
Cassava does not store as well as yams do. Apart from respiratory losses, cassava tissue becomes soft and rotten after exposure to the atmosphere for only a few days. Not much is known about the organisms responsible for cassava rot.
Industrial processing of cassava into gari has been developed, but this is still in its infancy. At present large numbers of village women still spend precious time with old, tedious, and relatively inefficient processing methods.
2. Plantains
Plantains constitute an important food crop in West Africa. Post-harvest physiological activities in plantains consist of ripening changes upon exposure to high environmental temperatures. Metabolic activities result in the utilization of carbohydrate and the release of carbon dioxide and oxygen. Ethylene also evolves in the ripening plantain. As ripening proceeds, the plantain becomes more susceptible to mechanical damage under poor storage conditions. In most of West Africa bunches of plantains are piled into open trucks for transportation to the markets and many of the plantains are crushed by the weight upon them. Therefore it becomes urgent to transport plantains in the green, unripe state as quickly as possible. Fungal attack on damaged plantains results in further wastage of the ripe crop. The techniques used to package and store bananas for the markets in countries with temperate climate could be utilized to protect plantains form these losses.
3. Fruits and Vegetables
Fruits and vegetables are perishable commodities and need special treatment and storage to prevent losses. In West Africa, there are no special storage facilities such as cold warehouses for most of these crops, and great losses are therefore sustained.
The living tissues of fruits and vegetables continue their metabolic activities during the post-harvest period. These activities are affected by the enzymes in the produce and also by environmental conditions such as temperature and humidity. At high temperatures most fruits and vegetables lose considerable quantities of their main nutritional ingredient, ascorbic acid (vitamin C). Exposure of fruits and vegetables to the sun in most African markets leads to destruction of vitamin content and other nutrients are also quantitatively reduced.
Infestation by microorganisms results in deterioration of fruits and vegetables during the post-harvest period, and urgent attention needs to be paid to this problem. Attack by microorganisms is made easy by the storage baskets used by farmers in West Africa. These baskets are rough on the inside, dirty, and are used constantly for storing and transporting various unwashed produce. The high rate of loss of fruits and vegetables from bruising and infection by microorganisms under these conditions needs to be investigated.
There is also the loss of produce caused by poor packaging during transport as well as in the markets. The sheer weight of the fruits and vegetables packed in large quantities crushes those on the bottom of the pack.
4. Cereals and Legumes
During the post-harvest period, cereal and legume crops are subject to losses due to a variety of causes- chemical changes, insect damage, rodent attack, and growth of microorganisms. By far the greatest losses occur through insect attack.
The grain weevil is responsible for major losses in weight and nutritive value because it bores into the grain and feeds on the endosperm. This reduces the weight of the produce and lowers its nutritive value. The damage can be so great that the crop may be reduced to "empty shells". These insects also lower quality by contaminating the grains. It is not unusual to find grain-sellers in most African markets picking out insect-damaged and mouldy grains from the pile exposed for sale. Mouldy grains are frequently found during storage and in the market places in tropical Africa, where the high humidity and temperatures are favourable to the growth of mould. Because protection of grains in storage, transportation, and marketing is negligible, mould infestation of most cereal grains and legumes is high.
Rodents are also responsible for large losses of grain. Large numbers of rodents, mainly rats, are found in the markets of West Africa, where they feed on the grains and contaminate them with their urine and feces. This means that in addition to the reduction in quantity, there is also deterioration in quality of the grain. Poor handling of grains also leads to spoilage and contamination with dirt. Because the protein-rich legumes grown in the country are infested and ruined in storage, the importation of canned baked beans, green peas, and a host of other legume products is now the rule in many African countries.
Clearly, the metabolic changes that occur in food crops cause a reduction in their nutrient content and therefore become a loss to the consumer. The growth, multiplication, feeding, and other metabolic activities of insect pests, rodents, and microorganisms also cause losses of valuable nutrients such as protein, carbohydrates, and vitamins, and usually make the food unfit and unsafe for consumption. Estimates of storage losses have been put between 10 to 30 per cent for cereal grains and about 20 to 65 per cent for legumes.
The losses of some nutrients such as ascorbic acid, or the rancid taste occasioned by the oxidation of fat, appear insignificant in the face of the physical food-crop losses. Though there is need to prevent some nutrient losses in food crops, and more studies are needed to determine those nutrient losses that take place during metabolism in the harvested crops, the greatest saving of food to provide for the increasing populations of developing countries can be achieved by preventing mishandling during transport and storage, and by protecting the food from attack by insects, rodents, and microorganisms. Proper processing of food is an urgent necessity in Africa. Food technology must be developed both by the study and improvement of indigenous processing methods, and by the adoption of modern technological activities that conserve nutrients and preserve food.
There was a brief discussion of the above paper at the workshop. The highlights in the discussion were as follows.
In developing methods for post-harvest conservation of foods, the need to look at the whole food system was emphasized. New technology must not be adopted blindly. For example, the sale of unripe fruits could pose problems both to the consumer and to the seller if the fruits are harvested too early and become rancid by the time they are bought.
If, however, they are collected, stored, and sold in a central place, the sellers may be deprived of some of their means of livelihood. Moreover, processing may create some problems in sorghum and rice by removal of the bran, with consequent losses in nutritive value.
The serious problem of mechanical damage that occurs when foodstuffs are being transported was stressed. The foods are bruised and often crushed during transportation, and this considerably adds to the losses. Physiological damage can occur when foodstuffs are stored under low ambient temperature after harvest. Surveys conducted in Nigeria indicate that losses occur during transportation because the produce goes through various distribution channels before reaching the market. Co-operative storage is a possible solution.
Attention was drawn to the fungal contamination of many varieties of foods, particularly by Aspergillus flavus, leading to production of aflatoxin.
A large body of information is available at the Federal Industrial Institute at Oshodi, Lagos, Nigeria. The major problem is the application of this information. Private industry is usually not interested in these developments because of the low profit incentive in applying them. Indeed, all activities related to food have been relegated to the women. There is need to educate and train people, both men and women, in this field. Several speakers stressed the need for governments and the international agencies to launch programmes on post-harvest conservation. For this purpose, studies should be undertaken where necessary to obtain more specific information on food losses. It was noted that the issue of food waste has been exhaustively treated in Nigeria in the publication, A Quantitative Analysis of Food Requirements-Supplies and Demands in Nigeria, 1968-1985, Ibadan University Press, 1972.
Post-harvest technological improvements may not help if the damage has occurred on the farm prior to harvest. The whole management of production and the marketing chain should be such as to minimize storage losses.
It was observed that some technology is not utilized because of communication difficulties, but some methods of storage are not suitable for adoption on a small-scale or home level. A whole range of technology needs to be developed to reach this category of people, as is being done in Japan where small equipment is specially developed for use on small farms.
It was emphasized that unless the government is committed to rural development, it cannot lay down a policy for this purpose. One problem is that some people believe that with technological innovations in food processing, the means of livelihood of traditional food processors is lost; hence, there is little or no interest in developing new processing methods.
It was noted that efficient methods of irradiation for the preservation of kola and yams have been developed in Nigeria. However, it was emphasized that the need is for appropriate technology to suit various levels. Universities and research institutes with good agricultural extension departments are needed to bring the findings of research to the common man at his level. The example of the Technological Consulting Centre at Kumasi, Ghana, where consultancy work is done on the basic requirements of the common man, was cited as an example. It is important to persuade the government to take an interest in the role of agriculture in improving human nutrition. And it was noted that the whole food-production system as well as social and economic factors are involved in determining this role.